This invention relates to submerged pipelines and process equipment, and more particularly, to deepwater pipelines.
Hydrocarbon discoveries are being made in increasingly deeper water. Frequently, tankers or liquefied natural gas (LNG) carriers are used to address deepwater hydrocarbon transportation needs. Pipelines and ancillary equipment are also used, but must be designed to withstand the extreme collapse pressures found at great depths. Conventional pipeline design relies on the strength of the pipe to withstand the high external water pressures in such service. The high strength is provided by using higher grade steels and greater wall thicknesses, both of which can increase the cost of the pipe material and the difficulty of installation. The design for conventional pipe systems, such as those used in the hydrocarbon transportation industry, is based upon the expected maximum internal operating pressure (PIO), as shown in FIG. 1, where the internal and external pressure of the pipe are equal and at atmospheric pressure during installation. In addition, for standard underwater pipe construction, the required pipe thickness (t) is based upon the external pressure head (PEH) of seawater at the installation depth, as shown in FIG. 2. The lowest internal pressure that the pipe is expected to encounter during installation, operation, and maintenance, which can be a vacuum, requires the pipe wall to be thick enough to prevent collapse of the wall due to the external pressure head. Hydrocarbon discoveries now exist in water depths where the optimum pipe diameter with a conventionally designed wall thickness may not be installable by existing pipelay vessels.
The existing technology to export the product from deepwater to shore needs to be improved. The transportation of hydrocarbon products from producing areas to markets across large and often deep bodies of water is also in demand.
Underwater pipelines have been installed by the S-lay or J-lay method as defined by the shape assumed by the pipeline as it is lowered from the installation vessel. The pipe can be transported to the installation site by towing, by ship-borne reel, or as individual joints. The current capabilities of these techniques are limited. The present invention addresses these limitations.